In conventional image generation systems, an original image is scanned to generate signals representing for example cyan, magenta, and yellow which, after processing, are fed to an exposing beam assembly which generates one or more exposing beams which are modulated in accordance with the scanned signals. The exposing beams impinge on a record medium which may be a light sensitive sheet or gravure cylinder. In the case of half-tone imaging, the control information is modified with half-tone dot information. Typically, the intensity of an exposing beam is controlled by a beam modulator.
In this context, a modulator comprises a device for controlling the transmission of radiation passing through the modulator in accordance with an applied control function. Such a modulator is hereinafter referred to as of the kind described. The response characteristic of the modulator to the control function commonly but not always exhibits a maximum or minimum. Typically, the response characteristic will be `U` or `V`-shaped and conveniently is symmetrical about hthe maximum or minimum. In this context `maximum` refers to maximum radiation transmission and `minimum` to minimum transmission or extinction. It should also be understood that "maximum" and "minimum" transmission relates to transmission in a particular direction since typical beam modulators cause incoming beams partially or wholly to deflect or diffract through angles determined in accordance with the applied control function.
A typical beam modulator of the kind described comprises an electro-optical modulator which responds to an applied voltage. Typically, the response characteristic has a sin.sup.2 form, that is the intensity of light transmitted varies in a sin.sup.2 fashion in response to applied voltage.
In some cases, the response characteristic of conventional modulators is not constant and tends to vary and, in some cases, drift over fairly short time periods and due to temperature variations. Typical drifts are in the order of tens of volts. Where a number of such modulators are mounted together, such as in the Crosfield Magnascan 645 system, variations greater than about 2 volts in response characteristics can lead to the generation of moire patterns in some colour separations which is undesirable. More commonly, optical components upstream of the modulators cause variations in the beams incident on the modulators and these variations can be compensated for by adjusting the transmission characters of the modulators.
In our earlier U.S. patent application Ser. No. 727,325 we described methods and apparatus for determining for each beam modulator the position of beam extinction or maximum beam transmission. Although these methods are satisfactory they are carried out on each modulator individually and in general the response characteristic between beam extinction and maximum beam transmission is not the same for each modulator. In the past manual balancing of the beam modulators has been carried out. This is a time consuming procedure and is not accurate since it relies on the skill of the field engineer.